Since Troxler's original observation in 1804, fading and filling-in phenomena have aroused the interest of vision researchers. However, the question of why during prolonged fixation a luminous target becomes embedded in the background (i.e., fading) and why in a center-surround stimulus the center acquires the properties of the surround (i.e., filling-in) has been systematically studied only during the last 30 years.
Fading has been attributed to local adaptation and is comparable to a stimulus falling below threshold. Strict fixation is crucial; as soon as the eye moves, the percept reappears. This effect is attributed to the edge signal arising at the border of the target due to image shifts produced by micro-saccades. There is evidence that the edge signal is propagated to the interior of the stimulus area and in this way restores the brightness of a region that adapts very quickly. In free viewing involuntary eye movements counteract adaptation and ensure that our perception is sustained over time. Without eye movements, a stimulus that is stabilized on the retina disappears almost instantaneously from view.
Filling-in refers to the spread of brightness, color, or texture into the stimulus center from the surround. An natural example present in our eyes since birth is the blind spot (optic disk), which is filled-in with stimulus properties of the surround and therefore is never seen. We have found that filling-in of the blind spot requires little surround information. A thin red ring hugging the boundary will fill-in the “blind“ spot area uniformly and completely with color. Filling-in also applies to lesion scotomata, i.e. holes within the retina and visual cortex, which likewise are invisible.
Psychophysical and neurophysiological experiments suggest that filling-in is achieved by long-range horizontal interaction beyond the classical receptive field. Candidate sites are visual areas V1-3. Here, neuronal behavior suggests that the contour of a center-surround stimulus fades first, before the attributes of the surround invade the center rendering both perceptually uniform. Fading and filling-in may thus be considered a “repair” mechanism used by the visual system to supply information to the brain that otherwise would be lost.
Literature: Luiz Pessoa & Peter De Weerd (eds.). Filling-In: From Perceptual Completion to Cortical Reorganization. Oxford University Press 2003